one of my new builds giving me grief

timc

The new AR in .308 is not performing well. The action is short stroking and not quite ejecting the spent cases. I have readjusted the gas block now for the 3rd time and drilled out the gas port slightly just to make sure there was nothing in there leftover after the barrel was coated. I'll give her one more try next weekend to see if I did any good but just thought I would throw it out on here to see if there is anything else I should look for. I have built quite a few AR's and never had one give me this much grief! The barrel is a 20" DPMS.

shotgunshooter3

Good luck. I don't build uppers specifically for this reason.


Sent from my iPhone using Tapatalk

Teach

Are you running NATO-spec ammo, or reloads? You might be experiencing a situation where the case is staying expanded against the chamber too long. CETME rifles had a fluted chamber to reduce the drag on the case and prevent extraction/ejection problems.
Jerry

timc

shotgunshooter3 wrote: »

Good luck. I don't build uppers specifically for this reason.


Sent from my iPhone using Tapatalk

I will get it running, it will not beat me!
There is no gas leakage that I can see, hopefully drilling the port out will solve it. She is almost there and did have it cycle 4 rounds in a row the other day when testing so I know it was close.

timc

Teach wrote: »

Are you running NATO-spec ammo, or reloads? You might be experiencing a situation where the case is staying expanded against the chamber too long. CETME rifles had a fluted chamber to reduce the drag on the case and prevent extraction/ejection problems.
Jerry

These are hand loads that cycle well in my other .308 gas guns so they should run here too. I double checked distance to the gas port on the block compared to the port on the barrel and all is good there too.
Chamber issue could be causing some problems, I could look there too.

NCFUBAR

Could the gassing problem be on after the gas block? Like the gas key on the BCG not being fully set tight ... or maybe the gas tube end being slightly off center or something and dragging on the inside of the key?

timc

NCFUBAR wrote: »

Could the gassing problem be on after the gas block? Like the gas key on the BCG not being fully set tight ... or maybe the gas tube end being slightly off center or something and dragging on the inside of the key?

Unless it is leaking by on the gas block but I don't see any signs of it. The bolt moves smooth and I don't see anything that could be a problem with the gas key. I double checked everything again this afternoon when I took it apart again, everything looks right. If drilling the port out didn't fix it I'll try another gas block.

Michakav

Are the gaps in the gas rings lined up?

orchidman

If it was a semi 12g that had the same problem I would first look at the chamber and polish it. Dont know whether that applies to AR's........

timc

Michakav wrote: »

Are the gaps in the gas rings lined up?

Hadn't thought of that, just took the bolt down and checked, all is good.

timc

orchidman wrote: »

If it was a semi 12g that had the same problem I would first look at the chamber and polish it. Dont know whether that applies to AR's........

Sometimes it does I may try it but all seems smooth.

Michakav

I am guessing you may have had an undersized gas port. Did you happen to gauge it? What type buffer are you running? If it is not a gas issue, it could be a buffer/spring issue. You will know after your next test.

Shows gas port sizes.....http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html

timc

Michakav wrote: »

I am guessing you may have had an undersized gas port. Did you happen to gauge it? What type buffer are you running? If it is not a gas issue, it could be a buffer/spring issue. You will know after your next test.

Shows gas port sizes.....http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html

Buffer and spring are good, I did not gauge the gas port but like you have am thinking that is the problem.

Farm Boy Deuce

DPMS must have really made big changes if they are making LR308 barrels with undersized gas ports now. My LR308 WILL NOT run with .308 Winchester ammo, just to much gas.

So I put an adjustable gas block on it a year ago and have not shot it since.

Lerchess

Yeah its highly unlikely a DPMS gas port is too small.

What gas block are you using?

The chamber might need to be reamed.

Might need a different weight buffer.

Gas key may not be tight.

Try hotter ammo / different mags.

Those are some things I would look at.

Farm Boy Deuce

When the gun short strokes is the fired case hanging halfway out the ejection port or hung up on top of the new shell? More like a double feed.

Michakav

knitepoet wrote: »

Try working both UP and DOWN some, that should, without a doubt, prove it it's over or under gaseed, depending on which way runs 100%

That's a great idea.

JasonMPD

Check your headspace. Mix-matched bolts and barrels for AR10 pattern rifles is something that happens.


Sent from my iPhone using Tapatalk

Gene L

Michakav wrote: »

Are the gaps in the gas rings lined up?

That's always the first thing to look at and the easiest fixed, but in all the ARs I've owned, I've never had that happen. Never heard of it happening, either, not for real.

timc

Farm Boy Deuce wrote: »

DPMS must have really made big changes if they are making LR308 barrels with undersized gas ports now. My LR308 WILL NOT run with .308 Winchester ammo, just to much gas.

So I put an adjustable gas block on it a year ago and have not shot it since.

Didn't say undersized, was thinking Maybe some of the barrel coating was built up in the gas port.

timc

knitepoet wrote: »

That was my first thought as well

FBD's comment got me to thinking, it COULD be overgassed to the point the bolt speed is so high the mag might not have a chance to push the next round up in time.

Also, dumb question probably, but have you ruled out a possible mag issue?

Not completely but I'm thinking not, I'll try a couple of different mags next time out.

timc

Lerchess wrote: »

Yeah its highly unlikely a DPMS gas port is too small.

What gas block are you using?

The chamber might need to be reamed.

Might need a different weight buffer.

Gas key may not be tight.

Try hotter ammo / different mags.

Those are some things I would look at.

Using a fairly cheap gas block so I'm thinking it may be the issue.
Changing from 16" to 20" barrel should not matter on buffer weight.
Gas key is good.
Will try different mags next outing

timc

Farm Boy Deuce wrote: »

When the gun short strokes is the fired case hanging halfway out the ejection port or hung up on top of the new shell? More like a double feed.

Not going far enough out to stove pipe or pick up the next round, going about 90% and hanging just before the ejection port but not deforming cases. Like its just not quite getting enough gas flow.

Farm Boy Deuce

A mis-drilled gas hole or some goo in there. Maybe knock the gas block off and gently run a drill that is gas tube sized down the hole by hand.

Shine a light down the gas tube hole and look at the bore of the gas block. I have never had an issue like this

BigDanS

If we assume the gas port, block and tube are lined up. we are left with the bolt gas tube / carrier key interface, the bolt rail / slide / lube system and then buffer.

From what you have said, I think your gas port, gas block and tube are likely correct, especially since you have rechecked it. Once the gas force is passed to the carrier gas key it needs to be tight enough to start the bolt backward, and the bolt has to slide sufficiently and not bind in order to initiate and complete the bolt cycle and this is where I believe your issue might be. "Run it wet" works better because it makes bolt cycle work with so much less friction. I would examine the gas key and make sure it is tight in the tube and then look at all the bolt and carrier friction points to make sure it isn't binding even slightly anywhere. Also look at your buffer tube and spring and make sure they aren't too strong.

IMHO

D

Lerchess

timc wrote: »

Using a fairly cheap gas block so I'm thinking it may be the issue.
Changing from 16" to 20" barrel should not matter on buffer weight. True but it might
Gas key is good.
Will try different mags next outing

A guy I know had similar problems that he solved by changing out the bolt release.

Building 308's can be a very different animal than .223.

timc

Farm Boy Deuce wrote: »

A mis-drilled gas hole or some goo in there. Maybe knock the gas block off and gently run a drill that is gas tube sized down the hole by hand.

Shine a light down the gas tube hole and look at the bore of the gas block. I have never had an issue like this

This is my first issue of all the mods and builds I've done.

timc

BigDanS wrote: »

If we assume the gas port, block and tube are lined up. we are left with the bolt gas tube / carrier key interface, the bolt rail / slide / lube system and then buffer.

From what you have said, I think your gas port, gas block and tube are likely correct, especially since you have rechecked it. Once the gas force is passed to the carrier gas key it needs to be tight enough to start the bolt backward, and the bolt has to slide sufficiently and not bind in order to initiate and complete the bolt cycle and this is where I believe your issue might be. "Run it wet" works better because it makes bolt cycle work with so much less friction. I would examine the gas key and make sure it is tight in the tube and then look at all the bolt and carrier friction points to make sure it isn't binding even slightly anywhere. Also look at your buffer tube and spring and make sure they aren't too strong.

IMHO

D

I'll be chasing all these one by one!